Process flow with wet etching for smooth sidewalls in silicon nitride waveguides

ABSTRACT

Aspects of the present disclosure are directed to process flow to fabricate a waveguide structure with a silicon nitride core having atomic-level smooth sidewalls achieved by wet etching instead of the conventional dry etching process.

RELATED APPLICATIONS

This application is related to and claims the benefit of U.S.Provisional Patent Application No. 63/078,825, filed Sep. 15, 2020,titled “Process Flow With Wet Etching For Smooth Sidewalls In SiliconNitride Waveguides,” the entirety of which is incorporated herein byreference.

TECHNICAL FIELD

The present disclosure relates to various structures and fabricationmethods for integrated photonics-based optical gyroscopes utilizingsilicon nitride waveguides with smooth sidewalls.

BACKGROUND

Gyroscopes (also referred to in short as “gyros”) are devices that cansense angular velocity. The applications of gyroscopes include, but arenot limited to, military, aircraft navigation, robotics, autonomousvehicles, virtual reality, augmented reality, gaming etc. Gyroscopes canbe mechanical or optical, and can vary in precision, performance, costand size. Since optical gyroscopes do not have any moving parts, theyhave advantages over mechanical gyroscopes as they can withstand effectsof shock, vibration and temperature variation better than the mechanicalgyroscopes with moving parts. The most common optical gyroscope is thefiber optical gyroscope (FOG) that operates based on interferometricmeasurements of optical phase shift due to the Sagnac effect (aphenomenon encountered in interferometry that is elicited by rotation).Construction of a FOG typically involves a coil comprising several turnsof polarization-maintaining (PM) fiber. Laser light is launched intoboth ends of the PM fiber coil so that two optical beams travel inopposite directions. If the fiber coil is moving, the optical beamstraveling in opposite directions experience different optical pathlengths with respect to each other. By setting up an interferometricsystem, one can measure the small path length difference that isproportional to the area of the loop enclosed by the turns of the fibercoil and the angular velocity of the rotating fiber coil. This pathlength difference is expressed as a phase difference (referred to as“phase signal”) between two counter-rotating beams.

Phase signal of an optical gyro is proportional to the Sagnac effecttimes the angular rotation velocity, as shown in the following equation:

Δϕ=(8πNA/λc)Ω

where, N=number of turns in the gyro,

A=area enclosed

Ω=angular rotation velocity

Δϕ=optical phase difference signal

λ=wavelength of light

c=speed of light

Fiber-based gyroscopes can provide very high precision, but at the sametime, they are of larger footprint, are very expensive, and are hard toassemble due to the devices being built based on discrete opticalcomponents that need to be aligned precisely. Often, manual alignment isinvolved, which is hard to scale up for volume production.

Present inventors propose replacing fibers with waveguide basedintegrated photonics components for cost-effective easy integration on asemiconductor platform which is much more promising for volumeproduction of gyroscopes. This application describes various structuresincluding silicon nitride (SiN) waveguide cores fabricated on a siliconplatform, as elaborated below. The SiN waveguide cores disclosed herecan have smooth sidewalls because of wet etching instead of theconventional dry etching methods that often result in micrometer ornanometer-range sidewall roughness, that can be detrimental to opticalperformance of the gyroscope.

SUMMARY

The following is a simplified summary of the disclosure in order toprovide a basic understanding of some aspects of the disclosure. Thissummary is not an extensive overview of the disclosure. It is intendedto neither identify key or critical elements of the disclosure, nordelineate any scope of the particular implementations of the disclosureor any scope of the claims. Its sole purpose is to present some conceptsof the disclosure in a simplified form as a prelude to the more detaileddescription that is presented later.

Aspects of the present disclosure are directed to process flow tofabricate a waveguide structure with a silicon nitride core havingatomic-level smooth sidewalls achieved by wet etching instead of theconventional dry etching process.

More specifically, a method is disclosed where a waveguide structure isfabricated by: forming a silicon nitride (SiN) layer on top of asubstrate with an oxide layer that acts as a lower cladding of thewaveguide, while the SiN layer, when patterned, acts as a core of thewaveguide; forming a cap layer on top of the SiN layer; patterning thecap layer by lithography and etching to form a patterned cap layercomprising a cap above the SiN layer, wherein a width of the cap issubstantially equal to a target width of the core of the waveguide; and,wet etching the SiN layer beneath the patterned cap layer to create thewaveguide core.

The waveguide structure can be used as rotation sensing element in anintegrated photonics optical gyroscope. The rotational sensing elementcan be in the form of a waveguide coil. The waveguide coil can bedistributed among multiple vertical layers, wherein light is coupledevanescently among the multiple vertical layers of the waveguide coil.Alternatively, the rotational sensing element is in the form of awaveguide-based microresonator ring.

Though for brevity, only the fabrication process is elaborated in thisparticular disclosure, Applicant incorporates by reference theearlier-filed patent application describing the single-layer andmulti-layer waveguide structures for optical gyroscopes, U.S. Pat. No.10,969,548, issued Apr. 6, 2021.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will be understood more fully from the detaileddescription given below and from the accompanying drawings of variousimplementations of the disclosure.

FIG. 1 is a schematic cross-sectional view showing silicon nitride (SiN)waveguide core layer deposited on oxide cladding, according to anembodiment of the present disclosure.

FIG. 2 is a schematic cross-sectional view showing a silicon dioxide(SiO₂) cap layer deposited on the SiN waveguide core layer, according toan embodiment of the present disclosure.

FIG. 3 is a schematic cross-sectional view showing patterning of theSiO₂ cap layer to appropriate width to form the SiN waveguide core,according to an embodiment of the present disclosure.

FIG. 4 is a schematic cross-sectional view showing SiN waveguide corefabricated by wet etching, according to an embodiment of the presentdisclosure.

FIG. 5 is an exploded schematic cross-sectional view of the SiNwaveguide core fabricated by wet etching, showing the smooth sidewalls,according to an embodiment of the present disclosure.

FIG. 6 is a scanning electron micrograph of the SiN waveguide core (witha hard mask on top) fabricated by wet etching, showing the smoothsidewalls, according to an embodiment of the present disclosure.

DETAILED DESCRIPTION

Aspects of the present disclosure are directed to methods of fabricationof compact ultra-low loss integrated photonics-based waveguide coreswith smooth sidewalls that can be done in large scale manufacturing.These waveguides can be used as optical elements on a planar photonicintegrated circuit (PIC), for example, in photonics integrated opticalgyroscopes. As discussed in the background section, the key tofiber-based optical gyroscopes' high performance is the long length ofhigh quality, low loss, optical fiber that is used to measure the Sagnaceffect. The present inventors recognize that with the advent ofintegrated silicon photonics suitable for wafer scale processing, thereis an opportunity to replace FOGs with smaller integrated photonic chipsolutions without sacrificing performance. Photonics based optical gyroshave reduced size, weight, power and cost, but in addition can be massproduced in high volume, are immune to vibration and have the potentialto offer performances equivalent to FOGs. When integrated opticalgyroscope is fabricated on a silicon platform, it is abbreviated asSiPhOG™ (Silicon Photonics Optical Gyroscope).

One key element of this integrated photonic solution is to produce verylow loss waveguide core made of silicon nitride (Si₃N₄) surrounded byoxide or fused silica claddings. The whole waveguide structure(including core and cladding) is sometimes referred to as SiN waveguidefor simplicity. The propagation loss in the SiN waveguides can be wellbelow 0.1 db/meter. This is a vast improvement over the currentstate-of-the-art SiN process with propagation loss in the range of 0.1db/centimeter.

FIG. 1 shows the first step in fabricating a SiN waveguide on aconventional silicon substrate. Specifically, FIG. 1 shows a substrate102, which may be a silicon substrate. The substrate 102 may have athickness ‘H’ of a standard wafer, e.g., the thickness can be 725 um.Note that the thickness of different material layers are not drawn toscale. However, in order to convey the idea that the substrate 102 ismuch thicker than the rest of the material layers shown in the figures,the discontinuity 101 is introduced in the middle of the layer 102 justfor visualization. The layers 104 and 116 can have a thickness ‘h1’ inthe range of 15 um on both sides of the substrate 102. Layer 104 acts asa lower cladding for the waveguide core layer 110. Waveguide core layer110, when patterned to the right dimension (as shown in FIGS. 4-5) canbe thought of one turn of the waveguide coil. Waveguide core layer 110can have a thickness ‘h’ and when patterned, a width of ‘w’.Non-limiting exemplary dimensions for ‘h’ can be 60-100 nm, and ‘w’ canbe 2-3 um. Waveguide core layer 110 is made of silicon nitride (SiN).Note that when layers 104 and 110 are formed on one side of substrate102, corresponding layers 116 and 118 are also formed on the other sideof the substrate 102, even though those layers may not be used forwaveguiding purposes. Alternatively, those layers can create waveguidesin a different layer, if necessary. An upper cladding layer 114 withthickness ‘h2’ in the range of 2-3 um may also be part of the structure.Both layers 114 and 116 can have the same material 120.

FIG. 2 shows the second step in fabricating a SiN waveguide core. A SiO₂cap layer 106 is deposited on top of the waveguide core SiN layer 110.

FIG. 3 shows the third step in fabricating the SiN waveguide core, wherethe SiO₂ cap layer 106 is patterned by etching to the appropriate width‘w’ (e.g., 2-3 um). The SiO₂ cap layer acts as a hard mask. It has beenexperimentally seen that wet etching the hard mask followed by wetetching the SiN layer gives the best sidewall roughness, because whenresist is used as mask and is dry etched, the sidewall roughness on theresist is “copied” down to the SiN layer underneath as well. In someembodiments, the cap layer can be dry etched too.

FIG. 4 shows the fourth step in fabricating the SiN waveguide core,where the waveguide core layer 110 is patterned by wet etching to theappropriate width underneath the patterned SiO₂ cap layer 106 with width‘w’ (e.g., 2-3 um), as shown within the oval dashed outline 400. Wetetching of SiN can be done by, for example, hot phosphoric acid. Thehard mask should be selectively resistant against the wet etchant.

FIG. 5 shows an exploded view of the SiN waveguide core 110 fabricatedby wet etching, showing the smooth sidewalls 510 and 512. The dimension‘x’ shows the recess underneath the layer 106 due to possibleover-etching (‘x’ is typically in the range of 20-25 nm on either side).The smooth sidewalls achieved by wet etching helps reducing the opticalloss during propagation within a gyroscope waveguide coil. The sidewallroughness achieved by wet etching is in atomic level, while the sidewallroughness achieved by dry etching is in the micrometer or nanometerrange, i.e. much higher roughness than the atomic-level smoothness.Depending on the waveguide core lateral dimension (e.g., thickness ‘h’),this smoothness can be a significant factor in dictating propagationloss and optical mode confinement, especially around waveguide bends.

After the wet etching, an upper cladding is deposited on top of theremaining hard mask above the SiN core. The remaining hard mask can actas a part of cladding and ensures that the interface between the uppercladding and the core layer has high integrity and strength to keep theoptical mode tightly confined.

In the foregoing specification, implementations of the disclosure havebeen described with reference to specific example implementationsthereof. It will be evident that various modifications may be madethereto without departing from the broader spirit and scope ofimplementations of the disclosure as set forth in the following claims.The specification and drawings are, accordingly, to be regarded in anillustrative sense rather than a restrictive sense. Additionally, thedirectional terms, e.g., “top”, “bottom” etc. do not restrict the scopeof the disclosure to any fixed orientation, but encompasses variouspermutations and combinations of orientations.

1. A method for fabricating a waveguide structure, comprising: forming asilicon nitride (SiN) layer on top of a substrate with an oxide layerthat acts as a lower cladding of the waveguide, while the SiN layer,when patterned, acts as a core of the waveguide; forming a cap layer ontop of the SiN layer; patterning the cap layer by lithography andetching to form a patterned cap layer comprising a cap above the SiNlayer, wherein a width of the cap is substantially equal to a targetwidth of the core of the waveguide; and wet etching the SiN layerbeneath the patterned cap layer to create the waveguide core.
 2. Themethod in claim 1, wherein the target width of the waveguide core is 2-3micron.
 3. The method of claim 1, wherein a material of the cap layer issilicon dioxide.
 4. The method of claim 1, wherein the cap layer acts asa hard mask during the wet etching.
 5. The method of claim 4, whereinthe wet etching is performed using hot phosphoric acid.
 6. The method ofclaim 4, wherein selectivity between the hard mask and SiN during thewet etching controls dimension of lateral recesses underneath the caplayer that is created in the patterned SiN layer.
 7. The method of claim4, wherein a sidewall roughness achieved by the wet etching is in anatomic level.
 8. The method of claim 7, wherein the sidewall roughnessin the atomic level is substantially less than nanometer range.
 9. Themethod of claim 1, wherein a thickness of the SiN layer is in the 60-100nm range.
 10. The method of claim 1, wherein the patterned cap layeracts as a part of an upper cladding of the waveguide.
 11. The method ofclaim 10, further comprising: depositing additional oxide layer on topof the patterned cap layer to a target thickness for the upper cladding.12. The method of claim 1, wherein the waveguide structure is used as arotational sensing element in an optical gyroscope.
 13. The method ofclaim 12, wherein the rotational sensing element is in the form of awaveguide coil.
 14. The method of claim 13, wherein the waveguide coilis distributed among multiple vertical layers.
 15. The method of claim14, wherein light is coupled evanescently among the multiple verticallayers of the waveguide coil.
 16. The method of claim 12, wherein therotational sensing element is in the form of a waveguide-basedmicroresonator ring.